The immune response to polysaccharide (PS) antigens is highly regulated and has several distinguishing features including restricted subclass, variable region gene usage, fine specificity, and avidity. Simple PS not conjugated to protein (such as bacterial levan (BL) or Neisseria meningitidis group C (MCPS)) elicit a thymus-independent (TI) response. Our earlier studies of the TI-response to BL in two strains of mice showed strain-specific differences in monoclonal antibody (mAb) V gene family usage, fine specificity, and immunization protocol. BALB/c mAb were primarily VHJ606/VK11 and cross-reacted with inulin (In). CBA/Ca mAb used VHJ606 and VK11 genes separately, and could cross-react with In, but primarily did not. DNA sequence and avidity studies indicated that primary BALB/c mAb in general were germline (V14A gene), used identical DH and JH, and were low avidity. BALB/c mAb to two injections of BL were somatically mutated and higher avidity. Oligomer hybridization to genomic DNA indicated that CBA/Ca do not possess the V14A germline gene used by BALB/c mAb. CBA/Ca mAb used a variety of DH and JH gene segments and were somatically mutated. Ongoing studies include site directed mutagenesis, chain recombination, and germinal center analysis in both strains. PS conjugated to proteins (such as MCPS coupled to tetanus toxoid (MCPS-TT)), on the other hand, elicit a different type of response termed thymus-dependent (TD). Previous studies indicated that immunization with MCPS-TT compared to MCPS resulted in isotype change, increased diversity and increased avidity in response to the TD form. Dot blot and Northern analyses of anti-MCPS mAb reveal that VH gene family usage is dominated by VHJ558 (the largest VH gene family) and appears to correlate with VH gene family size in a random fashion in response to either the TD or TI form. Fine specificity and VH family usage do not seem to correlate with the exception of VH3609 mAb which are specific for MCPS and are only found in response to the TI form. One VK23 gene has been found to combine with VH3609 to bind MCPS. The VH7183 and VGAM3-8 genes are only found in response to the TD form. An understanding of antibody diversity patterns and immunoglobulin gene structure and usage in response to both TD and TI antigens is essential for developing expertise in genetic engineering of mAbs.